Regex: Match everything except backreference

Question

I have the following example lines:

a_a
b_c

How (using grep/egrep) would I match the lines where the first letter

Answer 1

May be you are looking for this:

\b([a-z])\w+(?!(\1))([a-z])\b

it works for:

a_a and b_c

(tested with: The Regulator 2.0.3)

You can then adjust this regex to meet further your needs